As of this week, Tampa Bay Water has virtually drained its 15 billion-gallon reservoir. From now until the summer rainy season, it must rely on its two remaining sources of water: its sometimes troubled desalination plant and the dwindling supply in the underground aquifer.

"It's going to be a long couple of months waiting for the rainy season," Tampa Bay Water spokeswoman Michelle Robinson said Friday.

The regional utility expects to again ask the Southwest Florida Water Management District to impose the toughest watering restrictions in history on Pinellas, Pasco and Hillsborough county residents. Swiftmud turned down that request last month — but that was before the reservoir ran out.

The regional utility stopped taking water out of the reservoir Thursday. About 130 million gallons remain in the bottom — about a day's worth — but that's not enough to produce sufficient hydraulic pressure to push it through the pipe, Robinson said. The utility is adding a pump, but not until next year.

The reservoir, which covers about 1,100 acres in rural Fort Lonesome, is normally filled with water from rainfall, the Hillsborough and Alafia rivers and the Tampa Bypass Canal. But an ongoing drought, which started three years ago, has left all those sources depleted.

Meanwhile cracks in the reservoir walls have further hampered the water supply effort. To figure out what was causing them, utility officials lowered the water level last year. As a result, the reservoir was only half-full when Tampa Bay Water began tapping it last September.

Now, with no more water available from area rivers or the reservoir, the utility's general manager, Gerald Seeber, told board members this week that he is shutting down the $144 million surface water treatment plant in Brandon, which has been running since 2002.

The plant's employees, who work for a Tampa Bay Water contractor called Veolia, can keep busy doing maintenance projects, Robinson said. But the 66-million-gallon-a-day plant is likely to stay idle until June brings rainy weather again.

In the 1990s, when groundwater pumping drained swamps and dried up private wells, Swiftmud helped form Tampa Bay Water and agreed to help develop alternative water sources.

The regional utility then built its reservoir and its surface water treatment plant, as well as the 25-million-gallon-a-day desalination plant, which wound up being five years late and $40 million over budget when it finally opened last year.

In 1998 the utility pumped 147 million gallons a day from underground, damaging area lakes, rivers and wetlands. Thanks to the addition of those new water supply sources, by December that amount had dropped to 87 million gallons a day.

But the region's recent lack of rainfall has spurred the utility's 2 million water users to increase their lawn sprinkling. As a result, the utility expects to pump more than 100 million gallons a day in March, more than 140 million in April and to peak at more than 160 million gallons a day in May.

"I don't see where we've got a lot of choice," said Pasco County Commissioner Ted Schrader, who sits on the Tampa Bay Water board. "We've got to deliver water to the residents of the Tampa Bay area."

He conceded, however, that increased groundwater pumping could again harm the lakes and wetlands, not to mention increase the risk of sinkholes.

Fearing a costly violation of its pumping permit, last month Tampa Bay Water officials asked Swiftmud to tighten watering restrictions to the strictest limits in history. But Swiftmud officials said they thought the current restrictions needed more time to work. Their next meeting is at the end of the month.

15 billion Gallons the Tampa Bay reservoir can hold

130 million Gallons remain, about a day's worth

87 million Gallons a day pumped from underground in December

100 million Gallons of groundwater expected to be pumped a day in March

140 million Gallons of groundwater expected to be pumped a day in April